Defer flushing of SLRU files.

Previously, we called fsync() after writing out individual pg_xact,
pg_multixact and pg_commit_ts pages due to cache pressure, leading to
regular I/O stalls in user backends and recovery.  Collapse requests for
the same file into a single system call as part of the next checkpoint,
as we already did for relation files, using the infrastructure developed
by commit 3eb77eba.  This can cause a significant improvement to
recovery performance, especially when it's otherwise CPU-bound.

Hoist ProcessSyncRequests() up into CheckPointGuts() to make it clearer
that it applies to all the SLRU mini-buffer-pools as well as the main
buffer pool.  Rearrange things so that data collected in CheckpointStats
includes SLRU activity.

Also remove the Shutdown{CLOG,CommitTS,SUBTRANS,MultiXact}() functions,
because they were redundant after the shutdown checkpoint that
immediately precedes them.  (I'm not sure if they were ever needed, but
they aren't now.)

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (parts)
Tested-by: Jakub Wartak <Jakub.Wartak@tomtom.com>
Discussion: https://postgr.es/m/CA+hUKGLJ=84YT+NvhkEEDAuUtVHMfQ9i-N7k_o50JmQ6Rpj_OQ@mail.gmail.com

src/backend/commands/async.c

@@ -554,9 +554,8 @@ AsyncShmemInit(void)
 	 */
 	NotifyCtl->PagePrecedes = asyncQueuePagePrecedes;
 	SimpleLruInit(NotifyCtl, "Notify", NUM_NOTIFY_BUFFERS, 0,
-				  NotifySLRULock, "pg_notify", LWTRANCHE_NOTIFY_BUFFER);
-	/* Override default assumption that writes should be fsync'd */
-	NotifyCtl->do_fsync = false;
+				  NotifySLRULock, "pg_notify", LWTRANCHE_NOTIFY_BUFFER,
+				  SYNC_HANDLER_NONE);
 
 	if (!found)
 	{